In this sort of lighting apparatus, at least one organic electroluminescence element is driven at low drive voltage (e.g., about a few to 10 s V) in comparison with discharge lamps. The element is hereinafter also referred to as an organic EL element or an OLED (Organic Light Emitting Diode). Accordingly, since OLED drivers can be produced at low cost in comparison with prior art ballasts, various OLED drivers have been proposed.
For example, Japanese Patent Publication No. 2005-78828 discloses lighting apparatus. The apparatus includes an organic EL element(s), a converting means, a switching means and a controlling means. The converting means is formed of a full wave rectifying circuit and a smoothing circuit, and converts alternating-current power into direct-current power. The switching means has four transistors constituting a full-bridge circuit. This switching means produces an alternating electric current from the direct-current power to supply the element with a forward direction current and a reverse direction current alternately. The controlling means detects an electric current flowing through the element through a detection resistor connected in series with the element, and turns the transistors on and off based on the detected electric current. For example, the controlling means adjusts a duty ratio with respect to the transistors in accordance with a designated luminance value. Specifically, in case that the luminance is raised, the duty ratio is increased so as to lengthen a supply time of the forward direction current to the element. Conversely, in case that the luminance is lowered, the duty ratio is decreased so as to shorten a supply time of the forward direction current to the element. Besides, in order to extend life of the element and prevent noise from circuit parts and visible flicker, a switching frequency for turning the transistors on and off is set to a higher frequency than that of a commercial power source (e.g., 50 or 60 Hz), preferably 20 kHz or more.
Japanese Patent No. 3432986 discloses an organic EL display device. Before applying emission drive voltage across an organic EL element, the device applies auxiliary voltage to the element. The auxiliary voltage is lower than the emission drive voltage and higher than barrier voltage of the element.
Incidentally, as described in Japanese Patent Publication No. 2005-78828, if the switching frequency of a power source for an organic EL element(s) can be set to a higher frequency than that of a commercial power source, especially twice or more, preferably a few hundreds Hz or more, visible flicker can be prevented. Further, if the switching frequency can be set to 20 kHz or more, noise can be prevented. However, in case that the element is supplied with square-wave voltage of which polarity is periodically inverted, it is difficult to set the switching frequency to such a high frequency. That is, an organic EL element has a capacity component owing to the structure and therefore has the circuit structure that the capacity component is connected in parallel thereto. Because of this, if the switching frequency is raised as stated above, voltage polarity is inverted during charging the capacity component and then an electric charge of the component is discharged. Accordingly, there is a possibility that the element cannot emit enough light. For example, since the capacity component of an organic EL element that is a few centimeters square is generally within a range from a few tenth F to a few μF, the switching frequency can be limited to a few kHz or less. For that reason, a switching frequency for generating the square-wave voltage cannot be so high.